Decline and Fall of the Dinosaur Empire

New research suggests some groups of dinosaurs were already in decline before an asteroid hit Earth and eliminated most.

Troodon formosus, a meateater, was not in decline before the dinosaurs’ mass extinction.

AMNH/J. Brougham

Few topics in science spark such fascination as the extinction of dinosaurs about 65 million years ago, at the end of the Cretaceous epoch. An asteroid impact and massive volcanism were apparently to blame. But were dinosaurs already in decline before those catastrophes? New research suggests that the picture was complex, and that the answer is “yes,” at least for some groups of dinosaurs.

Paleontologist Stephen L. Brusatte of the American Museum of Natural History in New York City and three colleagues used a novel approach to explore this question. Instead of examining trends in taxonomic diversity by counting dinosaur species or genera, a method fraught with problems because of the unevenness of the fossil record, the researchers performed a quantitative analysis of “morphological disparity”—the variability of body structure within a certain group of animals. “Because skeletal anatomy is closely tied to diet, ecology, and behavior, it is often thought that disparity is a more biologically meaningful measure of diversity than simple counts of species richness,” says Brusatte.

The researchers’ analysis of seven major non-avian dinosaur groups, comprising nearly 150 species, shows that giants such as hadrosaurs and ceratopsids—large-bodied, bulkfeeding herbivores relatively close to the bottom of the food chain—were already in decline toward the end of the Cretaceous. Other dinosaur groups, including many carnivores and midsize herbivores, however, did not decline. One group, the sauropods, even appeared to be on the upswing. Interestingly, that group included giant herbivores that, unlike the hadrosaurs and ceratopsids, lacked advanced chewing abilities. The patterns unearthed by the paleontologists were geographically complex, with some trends dissimilar or even opposite in different world regions at the same time.

Would non-avian dinosaurs still be roaming the Earth had natural disasters not brought the Cretaceous to a close? Perhaps hadrosaurs and ceratopsids would be long gone, but many others might still be flourishing. (Nature Communications)